1. Field of the Invention
The present invention relates to white balance adjusting apparatus, and more particularly to a white balance adjusting apparatus in an image-sensing apparatus, such as a color video camera, for automatically adjusting white balance to correct the wavelenght distribution of light resulting from differing light sources. The correction is accomplished according to the color information signal within the image-sensing signal obtained from an image sensing device.
2. Description of the Background Art
In shooting an object using an image-sensing apparatus such as a color video camera, the wavelenght distribution of light illuminating the object from a light source differs by the type of the light source. For example, the blue components are intensive in light from a light source of relatively high temperature, whereas the red components are intensive in light from a light source of relatively low temperature. It is therefore necessary to correct the wavelength distribution of each light source in order to properly reproduce the color tone of the object. This correction is generally called white balance adjustment, where the gain of each color signal is adjusted so that the ratio of the amplitudes of the three primary color signals of red (hereinafter referred to as R), blue (hereinafter referred to as B), and green (hereinafter referred to as G) is 1:1:1 in shooting an achromatic object.
In conventional image sensing apparatus, the detection of the three primary color signals R, G and B is accomplished out according to light around the image sensing apparatus using a sensor provided for each color. However, white balance could not be adjusted correctly with such image sensing apparatus when the light source around the image sensing apparatus (for example, fluorescent light) differs from the light source illuminating the object (for example, the sun), as in the case where an outdoor scene is taken from inside a room.
Recently, a method called TTL (Through-The-Lens) is proposed in which white balance adjustment is carried out, without providing separated sensors, according to color difference signals R-Y and B-Y within the image sensing signal obtained from an image sensing device. Such a method is disclosed in Japanese Patent Laying-Open No. 62-35792, for example. This method is based on consideration that the object taken by an image sensing apparatus has various color area distribution (hereinafter referred to as the color distribution) and if this color distribution is averaged over a sufficient long time, the color components cancel each other to result in each color signal becoming "0", which is equivalent to a state of taking a completely white picture. According to this method, by controlling the magnitude of respective color signals so that the values resulting from integration of color difference signals R-Y and B-Y over one field period, for example, become 0. Consequently, the deviation of the color tone due to wavelength distribution of light of the light source can be corrected.
FIG. 1 is a block diagram showing an example of a conventional white balance adjusting apparatus using the TTL method. Referring to FIG. 1, light from an object (not shown) enters an image sensing device 2 formed of a CCD via a lens 1. The incident light is photoelectrically into an electric signal by the image-sensing device 2 and provided to a color separating circuit 3. Color separating circuit 3 extracts the three primary color signals of R, G and B from this electric signal. The extracted G signal is directly provided to a camera processing and matrix circuit 6. The extracted R signal and B signal are provided to camera processing and matrix circuit 6 via a gain variable R amplifying circuit 4 and a B amplifying circuit 5, respectively. Camera processing and matrix circuit 6 produces a luminance signal Y and color difference signals R-Y and B-Y according to the entered three primary color signals of G, R and B. The outputs are provided to a video circuit 7 where luminance signal Y and color difference signals R-Y and B-Y are subjected to the well-known process to produce a recordable video signal. This recordable video signal is provided to a video recording circuit not shown.
The two color difference signals R-Y and B-Y are applied to integrating circuits 18 and 17, respectively, to be integrated over a sufficient long time, for example over 1 field period of a video signal. The values resulting from integration are provided to gain control circuits 13 and 14. Gain control circuits 13 and 14 control the variable gains of B amplifying circuit 5 and R amplifying circuit 4 so that each value resulting from integration becomes 0. This results in the amplitude ratio for 1:1:1 of the three primary color signals G, R and B and adjustable white balance.
The conventional white balance adjusting apparatus of FIG. 1 corrects the irregularity of the wavelength distribution due to light of the light source, based on the consideration that colors cancel each other so that the reproduced picture can approximate a substantially white picture if the various color distributions of the object itself are averaged over a long period. A disadvantage of this method is that proper white balance regarding the object itself could not be achieved because the reproduced picture could not approximate a white picture even if the color distributions of the object included in the entire picture were averaged. This problem arises when the area ratio of the three primary colors within the picture is not equal, that is to say, when the color distribution is not equal, such as in the case where green lawn or a blue sky occupies a large area of the picture, or in the case where a human object wearing a red sweater is taken in a close-up manner. If the above mentioned white balance adjustment is applied to such an unbalanced state of white balance, the gain will be controlled so as to cancel the most intensive color in the image. In the case of a close-up of a person wearing a red sweater, white balance will be intense in blue, resulting in a problem that the color of the object itself cannot be properly reproduced on the screen.
This problem is described theoretically hereinafter. FIG. 2 is a graph showing the changes of two color difference signals R-Y and B-Y obtained in the case where a light of the light source illuminates a white color object or an object including each color on average as the color temperature of the light source changes. The ordinate indicates the red color difference signal R-Y whereas the abscissa indicates the blue color difference signal B-Y. As the color temperature of the light source changes, the color information of the screen, i.e., the obtained color difference signals vary only within a distribution range in the vicinity of a fixed locus (called the light source color temperature axis) crossing the origin, i.e. the white region, as shown in FIG. 2.
In ordinary image sensing situations, there are many cases where an object of chromatic colors having color information not in the distribution range occupies a large area of the picture. The obtained color information not within the distribution range, i.e. the color difference signals out of phase from the light source color temperature axis do not reflect the light source color temperature and are not appropriate as the color information for white balance adjustment. It is desirable that these color difference signals not be considered when the white balance is adjusted.
The light source color temperature axis crosses the origin (white color), as shown in FIG. 2, where the color difference signal of red is reduced as the color difference signal of blue increases (the fourth quadrant), and the color difference signal of red increases as the color difference signal of blue is reduced (the second quadrant). The first and third quadrants, not including the light source color temperature axis, do not reflect the light source color temperature and, therefore, are not adequate as the fundamental sources of information useful in the white balance adjustment. This means that when an object having significantly high chroma is included in the picture, the color distribution average of the entire picture does not show an achromatic color due to the effect of high chroma. The unnecessary white balance adjustment causes the white balance to be intense in the complementary color side of the high chroma color. Thus, the color of the object itself cannot be properly reproduced.
For example, as the case of image-sensing an object with high chroma, the situation in which a green portion such as lawn and plants occupying a large area on a screen is likely to occur. When the color distribution of an object is biased to green like this, both of the obtained color difference signals R-Y, B-Y become negative values, which are included in the third quadrant of FIG. 2. That is to say, such color signals have large phase difference from the light source color temperature axis and do not reflect the color temperature of the light source. Accordingly, such color difference signals should not be considered in the white balance adjustment.
In order to deal with such conditions, various methods have been introduced for removing the effect of large portions of an image being one color on the white balance adjustment. Typically, the methods operate by avoiding supplying to integrators 17 and 18, in the circuit of FIG. 1, color information from portions of the screen that include colors in the above described first and third quadrants or portions of the screen in which the luminance level exceeds a predetermined value.
When the image-sensed screen does not include all the respective colors on the average and, therefore, is not adequate for performing the white balance adjustment as a whole, it is necessary to avoid white balance adjustment on the basis of the color difference signals obtained from the entirety of the screen.
In such cases, it is required to determine whether various image information, including luminance level satisfy predetermined conditions in a select region of the screen. Consequently, either white balance adjustment is accomplished on the basis of the color information in the region selected or white balance adjustment is avoided on the basis of the color information obtained from the entirety of the screen.
However, when it is determined, in an alternative manner, whether various image information satisfy various conditions as described above, the effect upon the evaluation of the entirety of the picture exerted by a small change of the object in the vicinity of the region boundaries of respective conditions is large, so that the image reproduced on the monitor screen becomes unstable. In order to avoid such unstability, it is desirable to accomplish the regional processing by sub-dividing the various conditions into a set of intermediate conditions. However, processing with such an intermediate condition set is very difficult to be implemented in the aspect of the system capacity and processing ability of the system.